It has long been recognized that a hydraulic press, operated under a fluid pressure, is advantageous over a mechanical press because of its greater ability to effect a freely changeable slide motion and hence to meet with a wide variety of pressing configuration requirements. On the other hand, a hydraulic press has been found to be disadvantageous because it is slow in motion and hence low in productivity as compared with a mechanical press.
Thus, it has hitherto been the common recognition that while a larger sized press system can mostly be constructed of a hydraulic press, a smaller sized press system may only be served by a mechanical press.
As the capability of producing articles of a greater variety but in a smaller volume is increasingly required in the recent years, there has a demand more and more developed for a small sized hydraulic press better possessing such a production capability and yet operable at a higher speed.
Accordingly, the art has seen various proposals designed to enable a hydraulic press to be operated at a high speed.
For example, Japanese Unexamined Patent Publication No. Hei 7-124797 proposes a hydraulic press system in which a hydraulic cylinder assembly is provided in a piston rod thereof with an inner chamber that has a prefill valve in fluid communication therewith and that may be supplied with a pressure fluid when a slide is to be displaced rapidly in order to enable the slide to accomplish a given displacement in a shorter period of time, thereby permitting it to move at a greater speed.
Also, Japanese Unexamined Patent Publication No. Hei 4-55099 (Examined Patent Publication No. Hei 7-22840) discloses a hydraulic press system which makes use of a first cylinder of a smaller diameter that is provided in a second cylinder of a larger diameter and that may be supplied with a pressure fluid during a die close time to close the dies and clamp them together quickly, thereby permitting a pressing operation to be accomplished at a high speed.
Also, Japanese Unexamined Utility Model Publication No. Hei 7-3898 may be noted, setting forth another hydraulic press system in which a hydraulic circuit for delivering a pressure fluid into a ram cylinder includes a circuit section whereby the ram can be allowed to descend rapidly, thus permitting the system to act at a higher speed.
It is noted, however, that all these prior hydraulic presses intended to operate at a high speed have a hydraulic cylinder and a control valve for its operation coupled together by pipe conduits for pressure fluid distribution and are each so constructed that a pressure fluid of which the flow rate is controlled by such a control valve may be supplied via such fluid distribution conduits into the hydraulic cylinder. As a result, a change in compressibility of fluid in the pressure fluid distribution pipe conduits may occur and the responsibility in operation of the hydraulic cylinder by the control valve may deteriorate, resulting in an inability to form with a due precision. And such construction may cause the presence of a fluid resistance in these pipe conduits to disable a large volume of the pressure fluid to be supplied within a short period of time into the hydraulic cylinder, thus giving rise to an inconvenience imposing a serious limitation on the attempt to make a system to operate at a high speed.
In addition, these prior art presses are found to be inconvenient on account of the poor capability they possess, that arises from the fact that the hydraulic cylinder has the pressure fluid distribution pipe conduits mounted therearound requiring the presence of a large space upwards of its body portion, to make it difficult to reduce the entire press machine size and requiring a remedial maintenance operation at a height when a leak of fluid arises at the pressure fluid distribution pipe conduits, to deteriorate the maintainability of them.
It is accordingly an object of the present invention to eliminate inconveniences and disadvantages met in the prior art as discussed above and to provide a hydraulic press that is truly operable at a high speed, superior in the responsibility and rapidity of an operation, better in maintenance and small sized.